Hollow fiber membranes are generally provided in the form of “module” which can be readily and easily installed to a filtration facility. A hollow fiber membrane module has multiple strands or bundles of hollow fiber membranes, and various manifolds. The manifolds support and fix the multiple strands or bundles of hollow fiber membranes, and collect filtrates which permeate through pores of each hollow fiber membrane.
Such a hollow fiber membrane module is disclosed in, for example, Japanese Patent Application Publication Nos. Hei 7-155564, and Hei 7-178321, and Korean Patent Application Publication No. 2000-0050208. Further, a cartridge module of hollow fiber membranes, which can be easily mounted on a module mounting frame, is disclosed in U.S. Pat. No. 6,325,928 B1.
The important objects to be accomplished in designing a hollow fiber membrane module are “high degree of integration”, “easiness of mounting and dismounting”, and “easiness of cleaning”. The high degree of integration means that the maximized surface area of hollow fiber membranes is provided in a certain volume of module-occupying space. The easiness of mounting and dismounting means that troublesome manual working-efforts required to dismount and mount the hollow fiber membrane module from and on the module mounting frame is minimized. The easiness of cleaning means that a process for removal of the inter-membrane clogging of the hollow fiber membrane module which occurs during the operation of a filtration facility, is easily performed.
In the conventional hollow fiber membrane modules, the high degree of integration, the easiness of mounting and dismounting, and the easiness of cleaning were improved, but there still exists much room for improvement. Particularly, the further improvement in the aspect of the easiness of cleaning is needed.
The inter-membrane clogging phenomenon will be described below in detail. In the case in which the hollow fiber membrane module is submerged into wastewater containing high-concentration suspended solids, the suspended solids are deposited on and between the hollow fiber membranes. Thus, the effective surface area of the hollow fiber membranes which can be used for water permeation is reduced. This phenomenon is called “inter-membrane clogging”. As the inter-membrane clogging advances, the quantity of permeated water remarkably decreases, thus, the lifetime of the hollow fiber membrane module being shortened.
In order to relieve the inter-membrane clogging phenomenon, the following method is typically used. The hollow fiber membranes are shaken from the bottom of the hollow fiber membrane module through aeration of air, formation of a turbulent flow, generation of an ultrasonic wave or vibration, or the like during operating the filtration facility, so that the suspended solids deposited on and between the hollow fiber membranes are continuously removed. However, even if such a method is used, the inter-membrane clogging grows up to an unallowable level after the long-term operation.
Due to the recent development of methods for producing hollow fiber membranes, the lifetime-shortening of the hollow fiber membrane itself caused by deterioration of the material constituting the hollow fiber membrane has been much improved. Thus, if the inter-membrane clogging is removed, the hollow fiber membrane module can be reused several times. However, if the process of removing the inter-membrane clogging is very complicated, and takes much time and cost, the reuse of the hollow fiber membrane module is rather ineffective. For example, in order to remove the inter-membrane clogging, the clogged-portion of the hollow fiber membrane was treated by physical methods such as showering, and then, the hollow fiber membrane module was submerged and shaken again. However, such a conventional process took much time, but the removal level was not sufficient to reuse the hollow fiber membrane module.